I want to play the wiiware game ''Castlevania The Adventure ReBirth'' on my Nvidia Shield but it's very laggy and unplayable.
I don't understand why, because Contra ReBirth and Gradius ReBirth, very similar graphism, work perfectly.
I can play 3D games like Melee, Soul Calibur, Fire Emblem etc, so why a little 2D game can't have a good framerate?
Is there something i can do to run this game at full speed?
Maybe specific settings i guess.

It's very hard to tell from the visuals if a game is demanding for dolphin - due to having to emulate some specifics of the gamecube/wii hardware, some effects or programming methods tend to be much harder to emulate than others. The difficulty is more based on "How well it happens to map to modern PC hardware", rather than anything else.

It's not surprising that some games may look similar, but have significantly different demands when ran through dolphin.